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Sealing on a fabricated square face

Sealing on a fabricated square face

(OP)
Good Morning,

I have a question regarding sealing on a fabricated square face.

The part is formed / fabricated from sheet metal (2mm). Do you think it is feasible to create the flatness & surface finish on the green face to achieve a seal using a flat face gasket? Can you control the forming process to achieve parallel and flat face on all 4 sides? This isn't machined so will have the standard finish of rolled sheet. The corners are linished / polished to create a flat surface from the weld.

Many Thanks

RE: Sealing on a fabricated square face

it depends from side length and number of fasteners. If suitable You can use rubber foam gasket as much thick as You like

RE: Sealing on a fabricated square face

(OP)
Thanks for your response.

I understand that I will need to use a face gasket. The lid is / will be screwed down. I'm concerned about the manufacturing of the part and the ability to achieve flatness / parallelism & surface finish for a suitable seal.

Thanks again

RE: Sealing on a fabricated square face

There are quite a few considerations here. Is this going to be a one-off piece, or larger scale mfg? If only one piece, how much money do you have to spend on it? The more you spend, the more likely to be flat. If large scale, your maker will have time to figure out the best way to set up/jig etc. to get acceptable/repeatable results. How thick and pliable can your gasket be? If it's 5mm of soft foam, it can conform and forgive a significant deviation from flat/parallel. If it's 1mm of 70 durometer, you're gonna have to be dead nuts. If you have plenty of fasteners pulling it down, you can make up for a lot of warpage in the part at free condition. What is a "suitable seal"? Dust? Air? Water? Ingress or egress? Any pressure spec? (etc)

RE: Sealing on a fabricated square face

(OP)
Hi handleman,

As it stands this is low quantity (prototype), but eventually should be for higher quantity's. So as it stands this is going to have to be 1mm solid gasket i.e. Silicone / EPDM. We have 36 fasteners spaced around and are hoping to achieve 0.3mm compression on the face gasket. This has to create a full seal for all the ingress and egress that you mention. The lid uses M3 screws and is 5mm thick. Currently using C/SINK screws M3 x 12.

What do you mean by 'dead nuts'?

Great response with all the questions I was expecting.

Thanks

RE: Sealing on a fabricated square face

Go the other way.
But first.

What fluid is being sealed?
What pressure difference?
What liquid (gas ?) leakage is acceptable and safe? A furnace inlet filter or fan outlet cover has little ( if any) safety impact if a little leaks. Small leaks may have tremendous cost impact if the leaked material has economic costs, or enviro penalties associated with dripped fluids on the ground. Food or medicine requires near-perfect cleanliness at some times, but merely adequate seals in bulk handling material at other times in the process.
Bulk material may not leak at all.

If the opposing surface is flat and strong (like a flanged piece of casting or equipment), the light 2 mm thick piece of sheetmetal - even if machined perfectly somehow at great expense! - will twist and conform to the other surface. So machine the other surface adequately, add a soft gasket material compatible with your product and pressure, and let it bend.

RE: Sealing on a fabricated square face

(OP)
Hi,

The pressures are minimal (10 mbar). Temperatures up to 70°C. The 2mm sheet has a 'runner bar' underneath the sealing face allowing for extra thread length.

With regards to what is acceptable or safe leakage. We are trying to achieve the minimum possible leakage, as the liquid we use is a engineered fluid which evaporates very quickly. Hence the bare minimum leakage rate required.

If we have to use a solid rubber EPDM, do you think it is feasible to seal on a sheet metal formed surface? Is it possible with the correct 'jig' to achieve a flat parallel surface on a sheet metal formed part?

Many Thanks

RE: Sealing on a fabricated square face

You need to establish what your flatness requirement is. Then consult a fabricator and ask them if they can meet it with the design you provide. Learning from the sources on this forum is definitely important, but your most valuable input will come from the guys that have to make the part. Making them part of your design process is the most likely path to success.

RE: Sealing on a fabricated square face

Flat is nice.

Stiff is far more important.

"Bare minimum"is not a quantified engineering requirement.

RE: Sealing on a fabricated square face

Everything leaks, it's just a question of how much. You need to establish the acceptable leak rate before you can design a sealing system.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Sealing on a fabricated square face

(OP)
Hi,

Everything leaks - this is true. So with my acceptable leak rate figure I can then conclusively design a part to achieve this? My uncertainty isn't relative to whether the gasket and clamping force could achieve the leak rate, it is whether or not a sheet metal formed surface can be manufactured to create a capable finish for a sealing surface with regard to keeping things flat, parallel etc (lets say for a 1mm solid rubber gasket). Do you think its feasible to manufacture the 4 formed sealing faces flat relative to each other (0.1mm). The ideal scenario is I have fully machined faces to seal on nice thick flanges. However, this currently isn't an option.

Thanks

RE: Sealing on a fabricated square face

What's the longest dimension? It doesn't look too big to skim cut the flange as flat as it has to be. CNC air bending brakes can do some amazing things but I don't think you're gonna get a bent flange flat within .1mm over the whole surface. The depends on feed to the brake (distance), bend angle (true square with springback), and material uniformity (which affects springback), and there are flanges on every box. When you put heat in it with welding, somethings gonna go out.

RE: Sealing on a fabricated square face

A thin sheet flange point clamped on a squishy gasket will be wavy when installed.

Does not matter how perfectly flat things could be in the as-manufactured free state.

It will not stay flat when you tighten it.

RE: Sealing on a fabricated square face

How are the M3 fasteners to be tightened? The resulting clamp force could be as much as 300 lbs or so, but, might be a lot less.

How much force is needed to achieve the 0.3 mm compression?

For fun I'd be using the free statics FEA program in Solidworks or Creo/Wildfire to see what happens when a 300 lb load is applied to each bolt hole with the gasket replaced by the minimum sealing pressure the gasket maker says they need.

I agree, welded = bent and distorted.

If the cover does not have to be removed very often, formed in place gaskets can be a great solution.

With just about any gasket I'd be contemplating standoffs at each bolt hole to limit the compression.

If the fasteners are 3mm Ø, the width of the gasket inboard of the fastener hole ain't very wide. The gasket width at the two mystery holes near the corners is virtually nil.

RE: Sealing on a fabricated square face

Use a wet dispensed sealant and watch all your gasket compression and flatness problems disappear.

RE: Sealing on a fabricated square face

(OP)
HI,

The M3 screws are tightened using a pozi driver. There are compression limiters in the lid which bottom out at the desired 0.3mm compression. Formed in place gasket would be good, however it means scrapped part once the gasket becomes unusable?

I am guessing the answer is its very hard to achieve flatness and within 0.1mm between 4 faces created in 4 different operations on an cnc air brake.

If I could change to a thicker foam gasket it is what I would do and may have to do. This does have implications though for my design and isn't as straight forward as just replacing from solid to foam.

RE: Sealing on a fabricated square face

(OP)
Hi jgKRI,

Could you elaborate please? The lid needs to be removable.

Many Thanks

RE: Sealing on a fabricated square face

"The lid needs to be removable."
Once per shift, twice a year, every 3 years, every 100,000 miles ?

Formed in place gaskets don't destroy the parts. Some formulations can be tough to remove from the components.
Most Any gasket needs some width to perform ( seal) .. The attached image shows regions of the flange on your rendered image that look mighty narrow to me.

Here is a link to a paper gasket that seals quite reliably. The components are machined castings, but the area is generous.
https://www.nwmaicocz.com/maico-parts/pc/catalog/b...
There is such a thing as too much area, when the clamping contact pressure ( psi) is too low.
Makers of gasket materials have some pretty detailed info about such matters, to ensure the gasket is clamped everywhere

I expect There needs to be a compression limiter surrounding each fastener. A limiter midway between fasteners, or every now and then can make distortion matters worse.

"Great response with all the questions I was expecting."
If you were expecting some of these questions it would have been more efficient to provide answers in the OP.
It is not too late to pre-emptively answer all the questions you are still expecting.

RE: Sealing on a fabricated square face

Quote (Shawthing6927)

Could you elaborate please? The lid needs to be removable.

Sealant, not adhesive. Removal of your cover will not be a problem.

Wet sealant also means you don't have to tool and produce a custom gasket.

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